In recent years, wearable computers, such as computer display devices mounted on eyeglasses, including Google Glass™ made by Google Inc., have been developed and marketed; see also Japanese Patent Application Laid-open No. H11-98227. In a Google Glass device, an ultra-small display that uses a prism is arranged in front of the right eyeglass of the eyeglass frame (main body), enabling the Google Glass user to simultaneously view a virtual screen displayed on the display together with the real surroundings of the user. For example, by wearing this type of computer display device, the user can walk around town while viewing information, such as a map of the neighborhood, displayed on a virtual screen that appears to be floating in the peripheral vision of the user's right eye. In addition, for example, stores, restaurants, etc. in the vicinity of the user can display advertisements on the screen for the user.